The present invention is directed to polyaxial bone screws for use in bone surgery, particularly spinal surgery, and elongate connecting members that are at least somewhat plastically deformable. Such screws have a receiver or head that can swivel about a shank of the bone screw, allowing the receiver to be positioned in any of a number of angular configurations relative to the shank.
Many spinal surgery procedures require securing various implants to bone and especially to vertebrae along the spine. For example, elongate or longitudinal connecting members, such as solid rigid rods are often utilized that extend along the spine to provide support to vertebrae that have been damaged or weakened due to injury or disease. Such elongate members must be supported by certain vertebrae and support other vertebrae.
The most common mechanism for providing vertebral support is to implant bone screws into certain bones which then in turn support the elongate member or are supported by the elongate member. Bone screws of this type may have a fixed head or receiver relative to a shank thereof. In the fixed bone screws, the head cannot be moved relative to the shank and the rod or other elongate member must be favorably positioned in order for it to be placed within the head. This is sometimes very difficult or impossible to do. Therefore, polyaxial bone screws are commonly preferred.
Polyaxial bone screws allow rotation of the receiver about the shank until a desired rotational position of the receiver is achieved relative to the shank. Thereafter, a rod or other elongate connecting member can be inserted into the receiver and eventually the rod and the receiver are locked or fixed in a particular position relative to the shank.
A variety of polyaxial or swivel-head bone screw assemblies are available. One type of bone screw assembly includes an open head or receiver that allows for placement of a rod or other elongate member within the receiver. A closure top or plug is then used to capture the rod in the receiver of the screw. Thus, in such bone screws, the closure top or plug pressing against the rod not only locks the rod in place but also locks the bone screw shank in a desired angular position with respect to the receiver. A draw back to such a system occurs when the rod or other elongate connecting member is made from a material that is more flexible and may be more readily deformed or exhibit creep or viscoelastic behavior. Creep is a term used to describe the tendency of a material to move, flow or to deform permanently to relieve stresses. Material deformation occurs as a result of long term exposure to levels of stress that are below the yield or ultimate strength of the material. Rods and other longitudinal connecting members made from polymers, such as polyetheretherketone (PEEK), have a greater tendency to exhibit creep, than, for example metals or metal alloys. When a rod or other longitudinal connecting member exhibits creep deformation over time, the closure top may no longer tightly engage the connecting member. This in itself is not necessarily problematic. However, such loosening also results in loosening of the frictional engagement between the receiver and the bone screw shank that locks the angular orientation of the shank with respect to the receiver. Body movement and stresses may then result in undesirable pivoting of the shank with respect to the receiver causing mis-alignment, greater stress and further loosening of the various polyaxial bone screw components.